Two things to check: Either the cuff is too small for the patient (they make cuffs for obese patients), and the velcro isn't making enough connections to adhere to itself, or the velcro itself is all fuzzed up with lint or something and needs cleaning.
Make sure the cuff is on straight, not (gosh, how can I describe this?) sorta sideways overlapped like those lapel ribbons everybody wears for different fundraising causes. It won't read accurately if it's not wrapped straight.
There are also cuffs that are labeled as "long." You can use them for better wrapping around the arm. Pts who have huge arms often have short arms as well, so the extra large cuffs won't fit correctly. If you can find a long cuff, you can fully wrap around the upper arm without it going half way down forearm.
I agree - cuff's too small. And if you're VERY careful, you can use a pin to pull all the crud out of the rough side of the velcro to make it hold better.
My other thought is maybe it needs calibrated - in other words, is it inflating too much (past 200mmHg) for your patient population? That would also cause pop-offs.